Not applicable.
(1) Field of the Invention
This invention relates to welding methods for superalloys. More particularly, this invention is directed to a process for welding castings of nickel-base superalloys, and particularly gamma prime (xcex3xe2x80x2)-strengthened nickel-base superalloys, by which the occurrence of as-welded and post-weld heat treated cracking is reduced.
(2) Description of the Related Art
Nickel-base superalloys, such as gamma prime-strengthened nickel-base superalloys, are widely used to form certain high temperature components of gas turbine engines, including combustors and turbine nozzles (vanes) and buckets (blades). While high-temperature superalloy components can often be formed by a single casting, circumstances exist where superalloy components are preferably or are required to be fabricated by welding. As an example, in a high pressure turbine nozzle assembly in which the airfoils are formed of single-crystal (SX) superalloy and the inner and outer band castings are formed of a different superalloy or have a different microstructure, e.g., equiaxed, the airfoils would be assembled with the bands by welding or some other suitable method. Such components must generally be thermally stress-relieved after welding to relax residual stresses induced during cooldown following the welding operation. If formed of a gamma prime-strengthened nickel-base superalloy, heat treatment also provides stress relief by dissolution of a portion of the hardening gamma prime phase. Generally, the heat treatment parameters will vary depending on the superalloy composition, the amount of residual stress relief and dissolution required, furnace design, component geometry and many other factors.
Even if all appropriate precautions are taken, structural welds of gamma prime-strengthened nickel-base superalloy castings are known to form strain age cracks upon cooling from welding and post-weld heat treatment, such as during aging when the gamma prime phase is reprecipitated following solution heat treatment. The cause of these cracks is due at least in part to the residual stress produced during the welding and aging cycles. Cracking is particularly likely after welding castings formed of high gamma prime-strengthened nickel-base superalloys, which contain significant amounts (three weight percent or more) of aluminum and/or titanium. Aluminum and titanium are the primary elements forming the gamma prime phase that increases the strength, but also reduces the ductility, of nickel-base superalloys. High power levels can be required to weld such superalloys, which can promote cracking as a result of large volumes of the castings being consumed, creating high residual stresses from weld volume shrinkage, as well as higher levels of gamma prime formers in the weldment. The likelihood of cracking is further increased if the castings being welded are formed of dissimilar materials, e.g., with respect to chemistry, crystallography, coefficient of expansion, etc.
As a result of the tendency for cracking, components that must be fabricated by welding are often not formed of high gamma prime-strengthened nickel-base superalloys, but instead are formed from other high-temperature superalloys or are limited to being joined with fasteners or by brazing. However, the use of fasteners requires flanges, and brazing typically requires a large interface (faying surface), both of which result in increased weight. In addition, these assembly methods require machining operations that must be closely controlled in order to establish precise fit-up gaps. Finally, even if a crack-free weldment is successfully produced, various disadvantages can result, including a weak interface joint, a large joint gap, and compromised properties depending on the particular braze filler used.
In view of the above, it would be desirable if an improved process were available for welding nickel-base superalloys, particularly for the purpose of fabricating components formed of gamma prime-strengthened nickel-base superalloys.
The present invention generally provides a process for welding nickel-base superalloys, and particularly articles formed of gamma prime-strengthened nickel-base superalloys whose chemistries and/or microstructures differ. The invention reduces the incidence of cracking during cooling from the welding operation and subsequent heat treatments, while also producing a strong interface joint whose mechanical properties can be near that of the superalloys from which the articles are formed.
The method of this invention generally entails preparing the faying surface of at least one of the articles to have a cladding layer of a filler material. Depending on the properties required of the final welded assembly, the filler material may have a composition that is different from both of the articles, or the same as one of the articles. The cladding layer is preferably machined to promote mating of the faying surfaces, after which the faying surfaces are mated and welded together.
According to the invention, the welded assembly is free of thermally-induced cracks following cooling from the welding temperature. The assembly also remains free of cracks following subsequent heat treatments. The crack-free welds made possible with the invention are attributed to the cladding layer, which has been shown to reduce the likelihood of thermally-induced cracks in welded components formed of high gamma prime-strengthened nickel-base superalloys for use in the most severe operating environments of a gas turbine engine. Cracking can be avoided where the articles welded together have dissimilar chemistries, e.g., low versus high gamma prime-strengthened nickel-base superalloys, and different microstructures, e.g., single crystal versus equiaxed. As a result, the present invention provides an improved process by which components having relatively complex geometries can be fabricated by welding together two or more articles formed of different superalloys.
Other objects and advantages of this invention will be better appreciated from the following detailed description.